Transmission of genetic information without any deleterious alterations is one of the most important tasks for a cell to achieve in every cell cycle. To cope with this task, cells have evolved systems that survey, alert, and repair potentially lethal DNA damage. However, in situations where such systems are impaired, DNA damage accumulates and causes genetic changes. Accumulation of genetic changes, which is defined as a genomic instability is frequently observed in various types of genetic disorders including cancers. Genomic instability has been documented as a preceding step for multiple inactivations of tumor suppressor genes and activations of proto-oncogenes. One type of genomic instability observed frequently in many cancers is gross chromosomal rearrangement (GCR). GCR includes translocations, deletions of chromosome arm, interstitial deletions, inversions, amplifications, chromosome end-to-end fusion and aneuploidy. Although little is known about the origin and mechanisms of GCRs observed in cancer cells, recent studies on genes mutated in inherited cancer predisposition syndromes have started to demonstrate that proteins that function in DNA damage responses, DNA repair, and DNA recombination, play crucial roles in the suppression of spontaneous and/or DNA damage-induced GCRs 1) The role of post-replication repair (PRR) proteins including Rad5 and Rad18 in suppression of GCR. The PRR resolves the stalled replication folks. When replication machinery encounters a damaged DNA template, Rad6 and Rad18 monoubiquitinate Proliferating Cell Nuclear Antigen (PCNA) on lysine 164. Monoubiquitinated PCNA switches a replicative DNA polymerase to an error-prone translesion synthesis (TLS) DNA polymerase such as a DNA polymerase encoded by REV3/REV7 or an error-free TLS DNA polymerase eta encoded by RAD30. Accumulating evidence suggests a relationship between mutations in genes encoding PRR proteins and genomic instability. For instances, the targeted mutation of the mouse RAD18 gene in embryonic stem cells increased genomic instability including sister chromatid exchange, homologous recombination and illegitimate recombination. Mutations of XPV, the mammalian homolog of RAD30, that encodes TLS polymerase eta, were frequently found in xeroderma pigmentosum variant syndrome. Last year, we have shown that mutation of RAD5 or RAD18, ubiquitin ligases (E3) increases the de novo telomere addition type of GCR. The GCR suppression by Rad5 and Rad18 is dependent on the poly-ubiquitination of PCNA. In contrast, the SUMOylation of PCNA by Siz1 SUMO ligase at the same lysine residue is required for the generation of GCR. Inactivation of the homologous recombination pathway or the helicase, Srs2 reduced the elevated GCR rates by the rad5 or rad18 mutation. GCRs are therefore likely to be produced through the restrained recruitment of the HR repair pathway to the stalled replication forks. 2) The role of Rad1-Rad10 for the formation of GCR. Previously, we have demonstrated many mechanisms for suppression of GCR formation in yeast. However, pathways that promote the formation of GCRs are not as well understood. Therefore, we wanted to find genes responsible for the GCR formation in the absence of proper DNA repair. We found that the Rad1-Rad10 endonuclease, which plays an important role in nucleotide excision and recombination repairs, has a novel role to produce GCRs. Rad1 and Rad10 were first identified as members of the RAD3 epistasis group required for nucleotide excision repair (NER) of ultra-violet (UV)-damaged DNA in the yeast Saccharomyces cerevisiae. The human homolog of the RAD1 gene, XPF, is frequently mutated in xeroderma pigmentosum (XP), a cancer prone syndrome. Mutation of the RAD10 mammalian homolog in murine cells, ERCC1, generates a typical XP phenotype, including extreme sensitivity to UV light and a deficiency in NER. A mutation of either the RAD1 or the RAD10 gene reduced GCR rates in many GCR mutator strains. The inactivation of Rad1 or Rad10 in GCR mutator strains also slightly enhanced methyl methane sulfonate sensitivity. Although the GCRs induced by treatment with DNA damaging agents were not reduced by rad1 or rad10 mutations, the translocation and deletion type GCRs created by a single double strand break are mostly replaced by de novo telomere addition type GCR. Results presented here suggest that Rad1-Rad10 functions at different stages of GCR formation, and that there is an alternative pathway for the GCR formation that is independent of Rad1-Rad10. 3) Characterization of MRX (Mre11-Rad50-Xrs2) complex for its role of suppression of GCRs. Mutation in any of genes in the MRX complex (MRE11, RAD50 or XRS2) causes sensitivity to alkylating agents and ionizing radiation, defects in mitotic and meiotic recombination and NHEJ and also results in a decrease in telomere length. The fact that the mammalian MRX complex equivalent forms foci at the site of DNA damage suggests that the MRX complex functions in cell cycle checkpoints and DNA repair in mammalian cells. Furthermore, mutations of genes encoding these subunits have been identified in many cancer prone syndromes including Ataxia Telangiectasia Like Disorder (ATLD) and Nijmegen Breakage Syndrome (NBS). Null mutations in any of the MRX genes increased the GCR rate up to 1000 fold. However, because of the multiple functions and biochemical activities of the MRX complex, it is unclear which functions and what biochemical activities of the MRX complex are important for suppression of GCRs. To understand which function of MRX complex is the main function of GCR suppression, we initiated to generate different types of point mutations that specifically inactivate different MRX functions from last year. From these analyses, we found that at least three different activities of the MRX complex are important for suppression of GCRs. The nuclease activity of Mre11, an activity related to MRX complex formation and the telomere maintenance function of the MRX complex are important for the suppression of GCRs. Among these three functions, we further characterize the role of telomere maintenance function and its relevance for the suppression of GCRs. Lastly, the role each function for a specific GCR was further investigated.